1. Field of the Invention
The present invention relates to a communication system using a length shift keying (LSK) modulation method, and more particularly, to a communication system using an LSK modulation method which can minimize the structural change of the system and thus reduce its manufacturing cost by maintaining constant carrier signal power during modulation of carrier signals.
2. Description of the Related Art
Generally, spread spectrum communication is a method of spreading out a signal to be transmitted so that the signal has a much wider bandwidth than the original signal and transmitting the spread signal. Spread spectrum communication may be classified into a method of using a carrier signal of a narrow band that has a predetermined period such as a code division multiple access (CDMA) system and a method of using a carrier signal of a wide band. The method of using the narrow-band carrier signal is a method of performing frequency modulation so that the frequency band of information to be transmitted becomes narrower than the frequency band of the carrier signal. The method of using a wide-band carrier signal is a method of performing a frequency modulation so that the frequency band of the information to be transmitted becomes wider than the frequency band of the carrier signal.
Sine waves or impulse signals have been mainly used as a carrier signal in such a spread spectrum communication method, and recently, a method of transmitting information using a chaotic signal has been proposed according to the recommendations of the IEEE 802.15.4a standard.
In the spread spectrum communication, the carrier signal to be transmitted is modulated by diverse methods so that a receiver can accurately judge the received data. Carrier signal modulation methods include an amplitude shift keying (ASK) modulation method that modulates the amplitude of a carrier signal, a frequency shift keying (FSK) modulation method that modulates the frequency of a carrier signal, and a phase shift keying (PSK) modulation method that modulates the phase of a carrier signal.
The ASK modulation method is a method of modulating the amplitude of the carrier signal with the signal length of the carrier signal kept constant, and changes the amplitude of the carrier signal according to data bits that express data with “0” or “1”. According to this ASK modulation method, when the amplitude of the carrier signal is modulated, the power of the carrier signal is increased.
The FSK modulation method is a method of modulating the frequency of the carrier signal with the signal length of the carrier signal kept constant, and increases or decreases the frequency according to information bits, so that a receiver can discriminate the information bits according to the frequency. According to the FSK modulation method, however, when the frequency of the carrier signal is increased, the power of the carrier signal is also increased.
The PSK modulation method is a method of modulating the phase of the carrier signal to be transmitted by 180° according to the information bits. According to the PSK modulation method, when the phase of the carrier signal is modulated, the power of the carrier signal is increased in the same manner as other modulation method as described above.
As described above, according to the ASK modulation method, the FSK modulation method, and the PSK modulation method, the amplitude, the frequency, or the phase of the carrier signal is modulated so that the receiver can discriminate the received data, and the power of the carrier signal is increased.
However, since the power of the carrier signal that can be received by the receiver is generally fixed, the capacity of the receiver should be increased in order to receive the carrier signal having a high-level power. In addition, since the hardware construction of the receiver should be changed according to the modulation method for modulating the amplitude, frequency, or phase of the carrier signal, the hardware construction of the receiver becomes complicated and the manufacturing cost thereof is increased.
Accordingly, a method is needed that can modulate the carrier signal without increasing the power of the carrier signal and a method that can reduce the change of the receiver construction required for different modulation methods.